1. Field of the Invention
The present invention relates to a circularly polarized antenna device for transmitting and receiving circularly polarized electric waves.
2. Description of the Related Art
FIG. 9 is a perspective view of an example of a circularly polarized antenna device. A circularly polarized antenna device 30 is used, for example, in DAB (digital audio broadcast) systems in order to transmit and receive circularly polarized electric waves. The antenna device 30 comprises, for example, a circularly polarized antenna unit 31, a feeder circuit board 32, a feeder circuit (not shown), and a shield case 33. The circularly polarized antenna unit 31 comprises a rectangular parallelepiped dielectric base 35 and a circular radiation electrode 36.
More specifically, as shown in FIG. 9, the circularly polarized antenna unit 31 is constructed by forming the circular radiation electrode 36 onto the top surface of the rectangular parallelepiped dielectric base 35. With the bottom surface of the dielectric base 35 serving as a mounting surface, the circularly polarized antenna unit 31 is disposed on the top surface of the feeder circuit board 32. The feeder circuit for supplying electrical power to the radiation electrode 36 is formed on the bottom surface of the feeder circuit board 32. A plurality of feeder pins 37 which connect the feeder circuit and the radiation electrode 36 so that they are in electrical conduction are disposed so as to pass through the feeder circuit board 32 and the dielectric base 35. The shield case 33 for shielding the feeder circuit through a gap is provided on the bottom surface side of the feeder circuit board 32.
In the circularly polarized antenna device 30, electrical power is directly supplied to the radiation electrode 36 from the feeder circuit through the feeder pins 37. The supplying of electrical power excites the radiation electrode 36 in order to transmit and receive circularly polarized electric waves.
As described above, in the circularly polarized antenna device 30 having the structure shown in FIG. 9, the circularly polarized antenna unit 31 is disposed on the top surface of the feeder circuit board 32, and the shield case 33 which covers the feeder circuit through a gap is disposed on the bottom surface of the feeder circuit board 32. Therefore, the circularly polarized antenna device 30 is bulky. Consequently, although, in recent years, there has been a demand for small/thinner circularly polarized antenna devices, it has been difficult to meet this demand.
In addition, since, in the circularly polarized antenna device 30, the feeder pins 37 are disposed near the center of the dielectric base 35, it is difficult to carry out an aligning operation for properly connecting the feeder pins 37 and the feeder circuit on the bottom surface of the feeder circuit board 32 so that they are in electrical conduction. Further, since, in the circularly polarized antenna device 30, the feeder pins 37 are disposed near the center of the feeder circuit board 32, the output portion of the feeder circuit must be provided at the center portion thereof. A feeder circuit which has its output section at the center portion thereof is not easy to design, making it difficult to perform feeder circuit patterning.
The present invention has been achieved to overcome the above-described problems, and has as its object the provision of a circularly polarized antenna device which can be easily designed and produced, and which can be made smaller/thinner more easily. In addition, the present invention has as its object the provision of a radio communication apparatus using the circularly polarized antenna device.
To these ends, the present invention provides the following structures to overcome the above-described problems. More specifically, according to one aspect of the present invention, there is provided a circularly polarized antenna device comprising a circularly polarized antenna unit having a radiation electrode on a top surface of a substantially circular cylindrical dielectric base. The radiation electrode is used for transmitting and receiving a circularly polarized electric wave. The circularly polarized antenna unit is mounted to a top surface of a feeder circuit board with a bottom surface of the dielectric base serving as a mounting surface. In the antenna device, a recess is formed in the bottom surface of the dielectric base of the circularly polarized antenna unit. In addition, a feeder circuit for supplying electrical power to the radiation electrode is formed on an area of the top surface of the feeder circuit board covered by the recess of the dielectric base. Further, a shield for the feeder circuit is provided inside the recess of the dielectric base. Still further, a feeder electrode which connects to the feeder circuit so as to be in electrical connection therewith is formed on an outer peripheral side surface of the dielectric base so as to be separated from the radiation electrode. Still further, electrical power output from the feeder circuit is supplied to the radiation electrode through the feeder electrode by capacitive coupling.
Although not exclusive, a feeder wiring pattern for connecting the feeder circuit and the feeder electrode of the circularly polarized antenna unit so that the feeder circuit and the feeder electrode are in electrical conduction may be formed on the top surface of the feeder circuit board. In addition, a non-grounded area and a grounded area may be formed on the bottom surface of the dielectric base of the circularly polarized antenna unit. Further, an area of the bottom surface of the dielectric base with which the feeder wiring pattern is in contact may be defined as the non-grounded area, and a grounded electrode may be formed on an area of the bottom surface of the dielectric base excluding the non-grounded area.
When either one of the above-described structures is used, a feeder wiring pattern for connecting the feeder circuit and the feeder electrode of the circularly polarized antenna unit so that the feeder circuit and the feeder electrode of the circularly polarized antenna unit are in electrical conduction may be formed on the top surface of the feeder circuit board, and a groove may be formed in the bottom surface of the dielectric base of the circularly polarized antenna unit so that at least part of the feeder wiring pattern formed on the top surface of the feeder circuit board is covered through a gap.
When any one of the above-described structures is used, the dielectric base may be formed of a dielectric material having a dielectric constant which is smaller than the dielectric constant of the feeder circuit board.
According to another aspect of the present invention, there is provided a radio communication apparatus comprising any one of the circularly polarized antenna devices having the above-described structures.
In each of the above-described structures of the invention, a recess is formed in the bottom surface of the dielectric base of the circularly polarized antenna unit, the feeder circuit is formed on the area of the top surface of the feeder circuit board covered by the recess of the dielectric base, and a shield for the feeder circuit is formed inside the recess. In other words, in each of the above-described structures, the feeder circuit and the shields are accommodated inside the recess of the dielectric base. Therefore, the feeder circuit and the shield do not have to be provided on the bottom surface of the feeder circuit board, making it possible to correspondingly make the circularly polarized antenna device thinner.
In addition, in each of the structures of the present invention, the feeder electrode is formed on the outer peripheral side surface of the dielectric base of the circularly polarized antenna unit so as to be separated from the radiation electrode, and the electrical power output from the feeder circuit is supplied to the radiation electrode from the feeder electrode by capacitive coupling. In this way, the feeder electrode is formed on the outer peripheral side surface of the dielectric base, and the feeder circuit is formed on the area of the top surface of the feeder circuit board covered by the recess of the dielectric base as described above. Therefore, it is easier to connect the feeder electrode and the feeder circuit so that they are in electrical conduction, making it possible to prevent the occurrence of problems such as connection failures. Further, the output section of the feeder circuit is located at an end portion of the circuit. Such a feeder circuit is easy to design, thereby making it possible to perform feeder circuit patterning easily.